In a conventional refracting optical system using refraction of light, chromatic aberration is reduced by a combination of lenses made of glass materials having different dispersion characteristics. For example, an object lens of a telescope employs a combination of a positive lens made of low-dispersion glass and a negative lens of high-dispersion glass to correct chromatic aberration arising on the axis. However, it has been difficult to correct chromatic aberration sufficiently in the case where the configuration or number of lenses is limited or in the case where glass materials for use are limited.
Accordingly, it is known that chromatic aberration can be inhibited with a small number of lenses when a refractive optical element having a refractive surface and a diffractive optical element having a diffraction grating are used in combination. This makes use of the physical phenomenon that the direction of chromatic aberration arising for a light beam with a reference wavelength is reversed between a refractive surface and a diffractive surface of optical elements. In addition, by changing the period of diffractive gratings formed successively on a diffractive optical element, properties equal to those of an aspheric lens may be achieved.
However, a light beam incident to a diffractive optical element is divided into multiple light beams having different orders by diffraction. On this occasion, diffracted light beams having orders other than the design order form images at sites different from the site where the light beam having the design order forms the image, causing flare.
Accordingly, Patent Literature 1 discloses that optimization of the refractive index dispersion of each optical element and the shape of a grating formed on the boundary surface of optical elements enables high diffraction efficiency in a wide range of wavelength. By concentrating luminous flux in the range of wavelength for use into a specified order (hereinafter referred to as design order), the intensity of diffracted light having diffractive orders other than the design order is suppressed to a low level, resulting in flare being inhibited.
Specifically, BMS81 (nd=1.64, νd=60.1: manufactured by Ohara Inc.) and plastic optical material PC (nd=1.58, νd=30.5: manufactured by Teijin Chemicals Ltd.) are used in Patent Literature 1. Alternatively, COO1 (nd=1.5250, νd=50.8: manufactured by DIC Corporation), plastic optical material PC (nd=1.58, νd=30.5: manufactured by Teijin Chemicals Ltd.), and BMS81 (nd=1.64, νd=60.1: manufactured by Ohara Inc.) are used in Patent Literature 2.
Abbe number (νd) is calculated from the following Formula (3):νd=(nd−1)/(nF−nC)  Formula (3)(wherein nd represents refractive index at the d-line (587.6 nm), nF represents refractive index at the F-line (486.1 nm), and nC represents refractive index at the C-line (656.3 nm)).
The present inventors examined available or known optical materials of the diffractive optical elements and found the distributions shown in FIGS. 8A and 8B. FIG. 8A is a graph illustrating the distribution of Abbe numbers and refractive indices of general optical materials. FIG. 8B is a graph illustrating the distribution of Abbe numbers and secondary dispersion characteristics (θg, F) of general optical materials. The materials for the laminated diffractive optical element described in Patent Literature 1 are present also within the distributions in FIGS. 8A and 8B.
Patent Literature 1 also discloses use in combination of a diffractive optical element formed from a material having a relatively low refractive index dispersion and a diffractive optical element formed from a material having a high refractive index dispersion in order to achieve a configuration having high diffraction efficiency in a wide range of wavelength. The greater the difference in refractive index dispersion between materials having high and low refractive index dispersions, the higher diffraction efficiency and the wider field angle of an optical element formed therefrom are achieved. Accordingly, use of a material having a higher refractive index dispersion (or a small Abbe number) and a material having a lower refractive index dispersion (or a large Abbe number) is required for high-precision chromatic aberration correction.
Patent Literature 2 discloses an optical material having the relationship between refractive index (nd) and Abbe number (νd) represented by nd>−6.667×10−3νd+1.70, and the relationship between the secondary dispersion characteristic of the refractive index (θg, F) and Abbe number (νd) represented by θg, F≦−2νd×10−3+0.59. According to the disclosure, diffraction efficiency in the entire visible region is enhanced by satisfying these formulas.
Secondary dispersion characteristic (θg, F) is calculated from the following Formula (4):θg,F=(ng−nF)/(nF−nC)  Formula (4)(wherein ng represents refractive index at the g-line (435.8 nm), nF represents refractive index at the F-line (486.1 nm), and nC represents refractive index at the C-line (656.3 nm)).
Patent Literature 3 discloses use in combination of diffractive optical elements formed from a material having a high refractive index dispersion containing metal oxide fine particles such as ITO and a material having a low refractive index dispersion containing metal oxide fine particles such as ZrO2.